This invention relates primarily to image displays such as television cathode ray picture tubes and panel displays, and is particularly concerned with the quality of the body color of the unexcited viewing screens of such displays.
The viewing screens of image displays such as color television picture tubes of the aperture-mask type commonly comprise islands of light-emitting phosphor deposits surrounded by a black, light-absorbing matrix. This matrix is popularly referred to as "black surround," and a tube having black surround is well-known in the art as the black matrix tube.
The screen of the black matrix tube is best described in company with an illustration, comprising FIG. 1. This illustration is FIG. 4 of a disclosure by Fiore and Kaplan, U.S. Pat. No. 3,146,368, addressed to a cathode-ray tube with color dots spaced by black, light-absorbing areas. A triad of deposits of phosphor dots is shown, the dots comprising a red-light-emitting phosphor 10, a blue-light emitting phosphor 12, and a green-light-emitting phosphor 14. The name "black surround" is derived from the fact that the phosphor deposits are mutually separated from one another over the screen area and a light-absorbing material 16 is emplaced in the spaces between or around phosphor deposits 10, 12 and 14. Thus the full surface of the image screen, excluding the elemental phosphor areas, is formed of, or coated with, a black, light-absorbing material 16. The electron beams of the tube are larger in cross-sectional area than the phosphor dots; the landing area of the respective beams is indicated by the dash lines 17 encircling each of the phosphor dots. The concept of black surround made possible a cathode ray picture tube with a greater brightness and contrast than attainable with prior art tubes.
The phosphor deposits shown by FIG. 1 are dots; the deposits can as well be stripes; television cathode ray tubes having screens of this type are known as striped-screen picture tubes. A fragment of a striped screen with associated shadow mask is shown by FIG. 2. Strips of light-emitting phosphor comprising a red-light-emitting phosphor stripe 20, a green-light-emitting phosphor stripe 22, and a blue-light-emitting phosphor stripe 24 are shown as being deposited on the multi-color screen 26. The phosphor stripes are arranged in triads each associated with one of the apertures 28 of adjacent aperture mask 20. Three electron beams 30, indicated by the bracket, selectively excite associated phosphor stripes of each triad. Surrounding the phosphor stripes 20, 22 and 24, and interposed therebetween are stripes 32, comprising a light-absorptive black surround.
To fulfill the desired objective of maximum light-absorption, material 16 and stripes 32 are comprised of a material black in value and maximally non-reflective. The black surround comprises a deposit of material having the characteristic of being intensely black, or convertible to an intense black during tube processing, such as graphite, manganous carbonate, particles of carbon, or silver chloride.
Many processes are known in the art for depositing a black surround on the faceplate of image displays. A typical process is that disclosed by Mayaud in U.S. Pat. No. 3,558,310. An organic compound such as an unpigmented polyvinyl alcohol (PVA) and a sensitizing agent is printed on the faceplate from three color-center locations, leaving a pattern of clear stripes (or dots) which are intended to receive the triads of color phosphors. The faceplate is coated with a slurry containing graphite, for example. After the graphite has been dried, a hydrogen peroxide solution is used as a "developer" to remove the PVA deposits, leaving a pattern of black graphite surrounding clear glass areas on which are subsequently deposited the various color phosphors.
Continuing development to further increase the brightness and contrast aspects of tri-color picture tubes led to the structure described and claimed in U.S. Pat. No. 3,114,065 issued to Sam H. Kaplan. This patent discloses combining one or more of the three phosphor materials which make up the tri-color screen with a filter that is highly transmissive of the color or wavelength of light emitted by its associated phosphor material, but is otherwise an attenuator throughout the visible spectrum. The colorimetry can be controlled and an attractively low value of reflectance is provided which enhances contrast.
The color filter elements may comprise inorganic pigments or metal resinates, also termed "lusters." It is common practice to filter only the red-light-emitted and blue-light-emitting layers, while leaving green-light-emitting areas unfiltered. Such supplements to the phosphors as the aforedescribed filters may be termed "contrast-enhancing colorants" (and are so-referred to hereafter) when used in connection with picture tubes because of their ability to pass a certain portion of the spectrum of visible light while being opaque to other portions. Contrast is enhanced by the selective absorption of normal ambient light by the colorants, thus reducing the reflection of ambient light from the faceplate.
U.S. Pat. No. 3,886,394 to Lipp discloses an image display employing phosphor particles which are filter-coated. The filter particles are said to cover between 20% and 80% of the surfaces of the phosphor particles. It is alleged that by only partially covering the phosphor particles with filter particles, the transmission, absorption and reflection of light may be "tailored" to optimize the brightness and contrast of the display image in relatively bright ambients. Two embodiments are disclosed: one of which there is a single layer of phosphor particles coated as described and the other consisting of two layers comprising the coated phosphor particles, and over this a layer of phosphor particles which are uncoated.
Another example of contrast-enhancing colorant used in conjunction with a picture tube is the black stripe high-contrast color picture tube described in an article by Ikegaki et al (Toshiba Review, August 1976). A "graduated" pigment system is disclosed in which the concentration of pigment varies through the blue phosphor field, with the heaviest concentration of pigment being nearest the screen. A large increase in contrast and a slight increase in brightness over the standard Toshiba black-striped tube is said to be attained.
Colorants are defined as "those substances which modify the perceived color of objects, or impart color to otherwise colorless objects. They are characterized by having selective absorption and scattering of light so that they modify the spectral energy distribution of light falling upon them." (Principles of Color Technology, Billmeyer and Saltzman. New York: John Wiley and Sons, 1964.)
In the context of this disclosure, a "colorant" is defined as any material associated with the screen, including the faceplate itself, that contributes to the body color of the faceplate. "Body color" in turn is defined as the predominant hue of the faceplate under normal ambient light when the screen is unexcited. Included in the category of colorants are the phosphors and colored pigments and other filter materials which, in combination, may make up a viewing screen. In picture tubes that have no contrast-enhancing colorants, the body color of the unexcited screen is commonly a neutral gray. This gray aspect is attributable to the fact that phosphor materials, when unexcited, are generally colorless (with some exceptions), or if colored, the color is usually of very low intensity. Certain of the filters and pigments that are used as contrast-enhancing colorants, however, may be highly colored. If the screen contains an equal blend of the primaries of these colorants such as red, green, and blue, the body color will still appear as gray. This gray appearance is attributable to the fact that the viewing screen constitutes a "microstructure" of discrete particles which the human eye is unable to resolve into discrete color elements. So in effect, the eye integrates the primary colors and the faceplate appears as gray in totality. If one of the primary colors is not present, however, the body color will not appear as gray, but will appear as the product of the combination of the two remaining primary colors. For example, if only red and blue primaries are present, the body color will appear as a bluish-red, or magenta.
It is desirable that the body color of image display screens be neutral in aspect; that is, be of a neutral achromatic gray rather than a definite color such as the aforedescribed magenta. Any color other than a neutral gray is generally considered to be unacceptable.